selftests/seccomp: Update kernel config

seccomp_bpf.c uses unshare(CLONE_NEWPID), which requires CONFIG_PID_NS
to be set.

Cc: Kees Cook <keescook@chromium.org>
Cc: Shuah Khan <shuah@kernel.org>
Fixes: 6a21cc50 ("seccomp: add a return code to trap to userspace")
Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
Acked-by: Tycho Andersen <tycho@tycho.pizza>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20201202162643.249276-1-mic@digikod.net